(a) Field of the Invention
The present invention relates to a compact photographic camera lens system having an aspherical surface of short overall length, and more particularly, to a compact photographic camera objective using Leica size film, which is so designed that an aspherical surface is introduced into part of the lens system to minimize overall length as well as reducing the number of lens component of the lens system and where the aberrations of the lens system are properly corrected up to a field angle of around 63.degree..
(b) Description of the Prior Art
In general, a field angle of around 63.degree. is chosen for compact photographic camera objectives using Leica size film. In order to design lens systems of this kind as very compact systems having a telephoto ratio of 1.0 and lower, it is necessary to locate the rear principal point of the entire lens system before the first lens surface. For the purpose of satisfying this requirement, it is considered to adapt a lens type generally used as telephoto lenses comprising a front lens group designed as a converging lens system and a rear lens group designed as a diverging lens system. With the telephoto type lens system, however, it is extremely difficult to properly correct offaxial aberrations.
A lens system construction similar to that used in the present invention is disclosed in the Japanese Laid Open Patent Application No. Sho 55/1980-73014 in which a lens system is formed of only spherical lens systems. On the other hand, a lens system in which aspherical surfaces are formed on both sides of a fourth lens component is disclosed in the Japanese Laid Open Patent Application No. Sho 56/1981-59217 of the same inventor which has been made with the object of improving performance of the lens system in the former application in the marginal portions of the image plane. Specifically, a compact photographic camera lens system of short overall length is provided by introducing an aspherical surface at an extremely effective position for correcting aberrations in marginal portions of an image plane so that the aberrations are properly corrected. To achieve this purpose, the prior application indicates that the sufficient requirement is to introduce two aspherical planes into part of a lens system. However, the aspherical surface is disadvantageous in manufacturing due to difficulty of processing and inspection and accordingly presents an unavoidable expense. Therefore, it is apparently advantageous in manufacturing to reduce the number of aspherical surfaces to one. In this case, a problem is that, even if the number of aspherical surfaces to be introduced is reduced to one, how the same lens performance as that of a lens having two aspherical surfaces can be obtained.